1. Field of the Invention
This invention relates to an electrical power system. In particular, the invention addresses the level of harmonic distortions in electric power systems over a wide frequency range by the use of an 18-pulse DC supply.
2. Background
Many loads connected to AC distribution networks convert AC power into DC power. In order to reduce the harmonic currents generated by the conversion process a number of conventional techniques have been employed, such as passive filtering combined with 6-pulse rectification, active rectification, phase multiplication and the like. The phase multiplication method often used in aircraft and industrial electrical systems is known for its ruggedness and high reliability.
The main element of the phase multiplication method is a multi-phase transformer, supplied from a three-phase source and generating a multiphase AC supply for conversion into DC power, as exemplified in FIGS. 1A-F. FIG. 1A illustrates a block diagram of a conventional multiphase transformer having primary and secondary windings. A 3-phase AC source supplies the primary windings Vs1-Vs3. The primary is connected in a delta configuration and the secondary of the transformer includes both delta and wye connected windings that generate the appropriate phase shift and voltage for the 6-phase output V1-V6. FIG. 1B illustrates a phasor diagram of the respective winding. FIG. 1C illustrates a full wave rectification circuit that converts the 6-phase output of the secondary to a 12-pulse DC voltage. In the scheme of FIG. 1C, an interphase inductor is placed between the 6-pulse rectifying bridges in order to suppress cross-commutation between the bridges. The interphase inductor can be omitted in cases where the two 6-pulse rectifying bridges are supplied from two isolated sources. FIG. 1D illustrates the resulting waveform by superimposing Vd1 and Vd2. The resulting waveform has a period of xcfx80/6 and therefore results in a lower ripple voltage and current on the DC side. Multiphase rectification can be analyzed by superposing the effects of multiple 6-pulse rectification circuits supplied from multiple, phase-shifted, 3-phase sources, as shown in FIGS. 1D-F.
An example of such analysis applied to 12-pulse rectification is shown in FIGS. 1D-F, where the multiphase effect is obtained by using the 30-degree shift between voltages in delta and wye connections of the three-phase transformer, as shown in FIG. 1A. For instance, FIG. 1E illustrates the reflected primary side currents from each winding (i.e., delta and wye) as reflected back to the primary delta winding. The summation of the reflected secondary windings results in the total current Is1 as illustrated. FIG. 1F illustrates the secondary side currents that are the basis for the reflected primary side currents of FIG. 1E. These aspects of static DC power supplies and full wave rectification are well known in the art and will not be described further herein.
FIG. 2 illustrates typical waveforms of the primary side voltage 21 and current 22. The distortion in the primary side waveforms is a result of the switching and harmonics generated in the DC supply. Those skilled in the art will appreciate that there is generally and inverse relationship between the number of phases (pulses) on the secondary side and the total harmonic distortion (THD) on the primary side. Additional components such as passive filters, capacitors, chokes and the like can be used to further reduce the THD on the primary side.
Twelve-pulse rectification, commonly used in the aircraft and aerospace systems, generate total harmonic distortion in the range of 7% to 12%. The level of total harmonic distortion can vary from system to system due to the action of passive filtering usually added to enhance the performance of the power system.
In modern aircraft with electric power systems operating over a wide frequency range, it is desirable to keep the level of total harmonic distortion below 5% level. The level of total harmonic distortion as reflected on the primary side can be important because it represents a source of noise on the AC power source. This noise can effect sensitive equipment including causing the equipment to malfunction. An 18-pulse method can eliminate or reduce these effects by reducing the noise level on the AC power source.
FIGS. 3A-F illustrate an 18-pulse system wherein the figures correspond to the FIGS. 1A-F of the 12-pulse system to show the differences between the systems. Therefore, these figures are not described in detail except to illustrate comparisons between the 12-pulse and 18-pulse systems. The 18-pulse system is comprised of three 6-pulse systems phase shifted 40 electrical degrees. In contrast the 12 pulse system is comprised of two 6-pulse systems phase shifted 30 electrical degrees. In the 18-pulse system the voltages are balanced and an interphase inductor is not required, as is used in the 12-pulse system. Additionally, as shown in FIG. 3D the ripple voltage generated in the 18-pulse system is significantly less than the 12-pulse system of FIG. 1D. Correspondingly, the total primary side current Is1 of FIG. 3E has more steps of less magnitude than the primary side current Is1 of FIG. 1E. Consequently, the waveform of Is1 is less distorted in the 18-pulse system and less total harmonic distortion is generated on the primary side. Other well known differences between the 18-pulse and 12-pulse systems will be apparent to those skilled in the art and for brevity will not be described further herein.
In applications where the isolation between the AC power source and DC output is not required, a significant reduction in weight, volume (i.e., size), and cost can be achieved through the use of multiphase autotransformers instead of fully isolated transformers. In aerospace systems, where the component weight is one of the important design objectives, the use of autotransformers enables a new level of performance with respect to size, cost and weight.
One example of a multiphase rectification using an autotransformer is disclosed in U.S. Pat. No. 6,101,113. This patent describes a 12-pulse converter system with a 3-phase autotransformer with 4 windings per phase. The autotransformer is used to power two 6-pulse converter bridges connected in parallel with a large dc filter capacitor. The transformer rating is typically about 40% of the DC kW load. The voltage ratio is typically 1:1 so that the average dc output of a multi-pulse converter is generally the same as that of a conventional 3-phase bridge rectifier without transformer. A small single-phase transformer is used to block unwanted circulating currents between the two 6-pulse converters. Where necessary to further reduce high frequency harmonic currents, a 3-phase ac line reactor may be connected in series with the source of AC power. Where a smaller degree of harmonic reduction is acceptable, only 3 windings per phase are required on the transformer and raising the zero-sequence impedance of the autotransformer by means of an additional magnetic path eliminates the small single-phase transformer. This method can be also be applied to 18-pulse operation. However, the system requires the use of two zero-sequence blocking transformer (ZSBTs). Further, an additional impedance in one line is needed to ensure balance of the three, 3-phase bridge converter currents. Additionally, the arrangement of the windings is such that the input voltage is stepped up in a 1:1 design thus requiring additional turns for this application when compared to a 1:1 design that does not require a voltage step up.
The present invention utilizes an 18-pulse rectification system with autotransformer optimized for the lowest weight to overcome above-noted the prior art characteristics. The present invention provides an electrical power conversion system and a wye-connected autotransformer comprising three sections each spaced 120xc2x0 electrically apart, wherein each section comprises a main winding and a pair of phase shift windings. The main winding has a neutral end and source end, wherein the neutral end is connected to a floating neutral point and wherein the source end is connected to one phase of a three-phase power source. The pair of phase shift windings each comprise a second winding and a third winding connected in series, wherein one end of the second and third windings is connected to the source end of the main winding and wherein an other end the second and third windings is a phase of the wye-connected autotransformer that is phase shifted +/xe2x88x9240xc2x0 electrically from the source end. Each main winding has a same first number of turns, each second winding has a same second number of turns, and each third a same third number of turns. A ratio between the first, second and third number of turns is about 1:0.137:0.605, respectively.
In another embodiment, a wye-connected autotransformer according to the present invention comprises three sections each spaced 120xc2x0 electrically apart, wherein each section comprises: a main winding having a neutral end, a central tap and an output end, wherein the neutral end is connected to a floating neutral point, wherein the central tap is connected to one phase of a three-phase power source, and wherein the output end is a phase of the wye-connected autotransformer; and a pair of phase shift windings, wherein one end of each phase shift winding is connected to the central tap of the main winding and wherein an other end of each phase shift winding is a phase of the wye-connected autotransformer that is phase shifted +/xe2x88x9240xc2x0 electrically from the output end, wherein each main winding has a same first number and a same second number of turns as determined by the central tap and each phase shift winding has a same third number of turns.